Wood preservative composition



Patented Feb. I, 1938 UNITED STATES PATENT OFFICE woon rnasaava'nva coMrosITIoN Sonti Kamesam, New For 2 India est, Dehra Dim, British 3 Claims.

The object of this invention is to fix effectively and simultaneously, in one operation, a compound oi. copper and arsenic in wood, strawboards, palms and other materials containing cellulose and lignin so that when the latter, after drying, are exposed to moisture or running water, not only very little of any copper or arsenic is washed out of the material, but their relative and individual degrees'of leachability can be controlled within wide limits.

In this connection it may be mentioned that the British patent specification No. 2972/1912 states that metallic salts can be fixed in wood by the addition of water soluble chromium compounds. This claim has 'not been borne out to any considerable extent when zinc' chloride or sodium fluoride are used as the impregnating chemicals in my experiments. As far as I am aware, there has been no proposal made or mixture put on the market consisting mainly or solely of copper, arsenic and chromium, especially in the proportions I have found to be most suitable for wood preservation purposes. It is not only the toxicity 01' copper and arsenic against vegetable and animal organisms that is taken advantage of in this invention but also their optimum toxicity properties" corresponding to certain ranges in a high degree of fixation in wood. The latter fact coupled with the experimental fact that in the presence of chromium salts, copper commences to be fixed in wood much earlier than arsenic, tends to establish certain optimum properties between copper and arsenic salts in solution. Though containing copper,

' the preservative is neither decomposed by nor is it corrosive to steel and iron.

It has been proposed to use other organic and/or inorganic salts in admixture with chromic acid or chromium salts with or without arsenic compounds. By way of example, I may be allowed to refer 1) to the Indian specification No. 17,229 of 1930, in which it is proposed to use 45 parts of KZCI'ZO'I, 50 parts of NaF and 5 parts of dinitro-phenol with or without arsenic compounds, and (2) to the Indian specification No.

18,580/1932 in which it is proposed to use solely or almost solely arsenic and chromium salts. So far as I am aware, it has hereto not been proposed to use chromium compounds with cop-- per and arsenic compounds as main wood preserving ingredients, especially in a certain rela-.

tive proportion and to fix simultaneously both the metallic radicals of copperand arsenic. By the expression main ingredients, it is meant that the copper, arsenic and chromium compounds together should not be less than According to my invention, I propose to use chromium in the form of water-soluble chromium compounds along with water soluble copper and arseniccompounds as the main ingredients of the wood preservative.

If trivalent arsenic compounds are used, chromates or dichromates cannot be used in solution with them owing to the reducing action of the latter; chromic acid can, however, be employed, in which case the damage due to the corroding action on iron or steel vessels used in commercial wood preservation is enhanced. According to an important feature oi. my invention, chromium constitutes between 45 per cent to per cent of the total copper, arsenic and chromium (calculated in terms of the elements) present in solution. Within the above range, I have detected certain novel technical effects never observed before with regard to the simultaneous fixation of copper and arsenic in wood as will be evident from the description ,of the invention given below. The facts on which my present invention is based can be understood from the 7 following table in which are given the data, with Percentage of chemical (estimated on the original quantity injected) leached out alter 20,000 shakes in c. c. of water Percentage concentration of chemical in solution No. of experimont Potassium diehromate (KQCHO1) Copper Arsenic sulphate pentoxide (011504, (AS205;

- Copper sulphate (011804) (AS) Nil Nil The experiments of which the results are shown in the above table were carried out as follows: 4

Small pieces of air-dried semul woodiBombaa: malabaricum) were impregnated with wood preservative solution containing the chemicals, as

shown in the table, under test: in each case, a

known concentration of either or both of the chemicals was used. As the amount of the solution absorbed by the wood piece in each case was known by weighing the piece before and after the impregnation, the amount of arsenic, copper and/or chromium compounds injected into the woodcould be calculated.

The pieces after impregnation were dried and shaken in bottles containing c. c. of distilled water in each case. The amount of arsenic, copper and chromium compounds washed out in ments indicated in the table that copper is fixed.

to the wood corresponding to a relatively low proportion of chromium, whereas for the fixation of arsenic in wood, a relatively higher proportion of chromium salts appears to be necessary. Also, when a mixture of copper, arsenic and chromium salts are injected into wood, the chromium is used for the fixation of copper before any considerable fixation of arsenic can take place, during which process a further fixation of copper proceeds. The practical result of such a phenomenon is that if the chromium required for fixing copper as well as arsenicis slightly less than what it should be, the copper is not leached out, but there is a danger of -a considerable proportion of the arsenic being washed out of the wood or the like. Again, my experiments have shown that copper cannot be fixed in wood to the same veryhigh degree as arsenic without considerably reducing its toxicity so that the arsenic present should be as little as is necessary to resist fungus attack.

According to my experiments, the copper radical is fixed in wood more efilciently than arsenic for a certain quantity of chromium present and as wood-eating insects such as white ants contain more acid than fungi the copper will still be effective against such organisms, even against fungi, while arsenic will mostly cope with fungi against which it is very toxic even with a high degree or fixation in wood. The chemical basis for such an afllnity for copper in preference to arsenic by wood in the presence of chromium salts is not understood, but-this is a fact new to knowledge. On such a fact is built the best combination of copper and arsenic salts proposed in the invention. A useful formula for general conditions is as follows:.'75% CuSO4 5H20, 25% As: 0521-120, 1.25% KzCraOv.

My present invention is based on the fact that when soluble copper and arsenic compounds, in conjunction with chromium salts in aqueous solution, are used for impregnating different timbers, there is a certain critical ratio between, firstly, copper and chromium and, secondly, between arsenic and chromium above which only arsenic is washed out, or neither arsenic nor copper is washed out. In the case of hardwoods, containing gums, oleo-resins, a higher percentage of chroniium salts are, in general, necessary for the fixation of copper and arsenic in wood; I have made experiments with two coniferous and four hardwoods in this connection. Instead of employing equal proportions of copper sulphate and arsenic pentoxide, I have made experiments to test the fixation of copper and arsenic when the copper varies between 8% and 40%, and the arsenic varies between 8% and 50% of the total copper, arsenic and chromium per cent in solutionas elements. It may be noted that as regards toxicity to wood destroying fungi, copper and arsenic are to a large extent supplementary. If copper sulphate and arsenic pentoxide are used, generally very good results are obtained when copper sulphate (crystals) is used in twice to four times as much as arsenic pentoxide. In

other words, there are certain fungi like Lenzites' sp. to which arsenic is not very sensitive while copper is sensitive. There are again fungi like Coniophora sp. to which arensic is very sensitive, but copper is not quite so toxic. Again, in the case of insects, especially termites, copper appears to be almost astoxic as or even better than arsenic, especially when wood is protected from fungus attack by an efficient fungicide like arsenic.

In practice, wood may be preserved by the injection, under pressure (without having any effeet on the steel or on the preservative) of the composite solution in closed steel vessels, or dipping in open vessels or by brush or spray painting of the solution.

I claim:- I

1. A wood preservative consisting of about. 3 parts by weight of a copper sulphate, about 1 part by weight of a water-soluble arsenate, and

about 3-5 parts by weight of an alkali dichro 

